You know when cannabis hits the news you’re in for a bit of fun, and this week’s story about cannabis causing psychosis was no exception. The paper was a systematic review and then a “meta-analysis” of the data which has already been collected, looking at whether people who smoke cannabis are subsequently more likely to have symptoms of “psychosis” or diagnoses of schizophrenia. Meta-analysis is, simply, where you gather together all of the numbers from all the studies you can find into one big spreadsheet, and do one big calculation on all of them at once, to get the most statistically powerful result possible.

Now I don’t like to carp, but it’s interesting that the Daily Mail got even these basics wrong, under their headline “Smoking just one cannabis joint raises danger of mental illness by 40%”. Firstly “the researchers, from four British universities, analysed the results of 35 studies into cannabis use from around the world. This suggested that trying cannabis only once was enough to raise the risk of schizophrenia by 41%.”

In fact they identified 175 studies which might have been relevant, but on reading them, it turned out that there were just 11 relevant papers, describing seven actual datasets. The Mail made this figure up to “35 studies” by including 24 separate papers which the authors also found on cannabis and depression, although the Mail didn’t mention depression at all.

They also said that “previous studies have shown a clear link between cannabis use in the teenage years and mental illness in later life”. They then described some of these previous studies. These were the very studies that are summarised in the new Lancet paper.

But what was left out is as interesting as what was added in. The authors were clear – as they always are – that there were problems with a black-and-white interpretation of their data, and that cause and effect could not be stated simply. For ongoing daily users, as an example, it’s difficult to be clear that cannabis is causing people to have a mental illness, because their symptoms may simply be due to being high on cannabis all the time. Perhaps they’d be fine if they were clean.

It was also interesting to see how the risk was numerically reported. The most dramatic figure is always the “relative risk increase”, or rather: “cannabis doubles the risk of psychosis”, “cannabis increases the risk by 40%”. Because schizophrenia is comparatively rare, translated this into real numbers this works out – if the figures in the paper are correct, and causality is accepted – that about 800 yearly cases of schizophrenia are attributable to cannabis. This is not belittling the risk, merely expressing it clearly.

But what’s really important, of course, is what you do with this data. Firstly, you can mispresent it, and scare people. Obviously it feels great to be so self-righteous, but people will stop taking you seriously. After all, you’re talking to a population of young people who have worked out that you routinely exaggerate the dangers of drugs, not least of all with the ridiculous “modern cannabis is 25 times stronger” fabrication so beloved by the media and politicians.

And craziest of all is the fantasy that reclassifying cannabis will stop six million people smoking it, and so eradicate those 800 extra cases of psychosis. If anything, for all drugs, increased prohibition may create market conditions where more concentrated and dangerous forms are more commercially viable. We’re talking about communities, and markets, with people in them, after all: not molecules and neuroreceptors.

++++++++++++++++++++++++++++++++++++++++++
If you like what I do, and you want me to do more, you can: buy my books Bad Science and Bad Pharma, give them to your friends, put them on your reading list, employ me to do a talk, or tweet this article to your friends. Thanks!
++++++++++++++++++++++++++++++++++++++++++

AitchJay said,

wewillfixit said,

Gimpy said,

Is it just me or would the negative electrosensitivity trial have been better to cover this week? The cannabis stuff is all well and good but the controversy in the reporting is highly predictable and has been covered many times before. The real but apparently psychological symptoms of some electrosensitivities is far more interesting than the DM getting their facts wrong over an issue which they won’t debate seriously anyway.

malcolm said,

As a layman, what interests me in this is the direction of causality – maybe people with a propensity to developing schizophrenia in later life also have a propensity to smoke cannabis – “latent schizophrenia (if there is such a thing) causes cannabis use shock horror.”

When I did my philosophy degree years ago, we used to discuss the problem of causality, and simple correlation doesn’t equate to causality, even if there is temporal priority.

Not having read the research, only newspaper reports, how did the researchers answer the question of what was causing what, rather than just correlated with?

But the electrosensitivity issue is much more just an insight into how people work, how quackery thrives on bad science and how lobby groups interact. The numbers of people affected are quite small.

I would argue that the politicisation of this report could be far more damaging.

Government knee jerking and popularist moves are going to criminalise 10% of the population and this in itself may well do more damage than the relatively few who may be adversely affected by smoking cannabis.

Gimpy said,

10. the electrosensitivity issue is transferrable to many other spheres of woo practice though. The cannabis debate, when it deals with evidence and epidemiology, is less so as the arguments are about the specifics. Drug research is always politicised and usually to the detriment of the argument but drugs are an emotive issue for many not just a few like in electrosensitivity. Given that the pattern of misrepresentation and scaremongering over electrosensitivity is similar in pattern to that of drugs research but strong emotions are restricted to a few wouldn’t it be better to develop a strong argument against relatively non-emotive issues first then show that the exact same argument applied to emotive issues like drugs. IMHO obv.

odtaa said,

A recent study does show that smoking cannabis at university does lead to a small group of people getting delusions and ending up costing the tax payers billions. I believe they are called politicians.

Can I have research funds please to investigate if the cause of making people who have really good career prospects in the city etc decide instead to go into politics?

If it is true that cannabis does cause psychosis, often with an element of paranoia, does this explain the writing and readership of the Daily Mail. Maybe they should stop giving away DVDs and starting giving joints and trips to Amsterdam.

Can I have research funds to

1 Investigate the cannabis use of Daily Mail contributors and readership.

2 Investigate if reading the Daily Mail is more likely to cause a paranoid psychosis than cannabis use.

Two scare stories apparently missed by the Daily Mail is ‘passive’ cannabis smoking and secondly if cannabis smokers smoked near a reservoir would all the inhabitants of the town become murdering psychotics.

On a serious note if the politicians, who admitted smoking cannabis at university where charged and jailed then their careers would have been impossible.

desoi said,

Can someone please help me here? As a non-statistician, non-scientist and regular if infrequent toker (few joints every 6 weeks or so), how long should I have to wait on average for the psychosis to kick in?

Malcolm — they did mention some statistical techniques that the original authors of the papers in the study had used to correct for direction of causality.

Although as another ex philosopher, it left me none the wiser as to how they worked. Anyone know much about this stuff? Is it something that the stats wonks can tell you from the numbers alone, or do you need to look at pre-symptoms, or what?

as well as impenetrable stats voodoo on direction of causality, of course, some of the studies were birth cohort lifelong followup eg the dunedin study. they could assess for pre psychotic symptoms in early teens before cannabis use etc (sorry read it ages ago can’t remember full details).

on the interestingness of cannabis, tbh I was going to write on something completely different but the newsdesk asked for this. I didn’t think the news coverage was that dismal in general, the political interpretation is a bit dumb (class b = less use and less harm?) but not so much sci there. so anyway, that’s why it doesn’t have a central “point” like most of my stuff, I didn’t really have one to make. it was my experience of being polly filla to order. sure I could have come up with a direction for it if I’d had a min or two to think.

also, I take full responsibility even though it was tinkered by subs, but I’m worried they can take stats journalism prize away if you explain a risk as I did above in a sentenced with five subclauses and three grammatical errors.

banshee said,

“As a layman, what interests me in this is the direction of causality – maybe people with a propensity to developing schizophrenia in later life also have a propensity to smoke cannabis”

All too true – a number of people with serious mental illness “self-medicate” with cannabis and other illicit (and licit) drugs to help their mental state. Anecdotally (I know…) I’ve been involved in treating a number of cases where using CBD worsens psychosis and promotes admission into psychiatric services. Classical revolving door…..

Desoi said

“Can someone please help me here? As a non-statistician, non-scientist and regular if infrequent toker (few joints every 6 weeks or so), how long should I have to wait on average for the psychosis to kick in?”

The ABSOLUTE risk is pretty low – out of 1000 people who use cannabis 4 more will develop psychosis than if those 1000 did not use cannabis.

Individual susceptibility to mental illness isn’t something that is predictable – even if you use cannabis!

Robert Carnegie said,

By the way – I’m probably doing this the wrong way, but I look at stories here several times over, if the count of comments has gone up and someone said something interesting… by my count, several just lost one or more of the comments, apparently. Why is this?

I don’t quite see, though, that making a criminal drug more criminal than it already is would mean that “more concentrated and dangerous forms are more commercially viable.”

when something is illegal, then less dilute (and less harmful) forms are sidelined in favour of higher profit more concentrated products. you couldnt buy beer under prohibition, you cant buy coca leaves now.

i would also argue that crack is a function of the illegality of the market in various interesting ways: you can assess quantity and quality visually if you think people are ripping you off (whcih they will try to), it doesnt blow away on street corners, its more portable for ad hoc street saleforce, but most crucially, its more subjectively effective per dollar, etc.

calvin said,

“when something is illegal, then less dilute (and less harmful) forms are sidelined in favour of higher profit more concentrated products. you couldnt buy beer under prohibition, you cant buy coca leaves now”

Cannabis has become more or less legal over the last two decades; it’s now more potent than ever. Ecstasy’s legal status has remained largely unchanged, most of it is totally shit now. When a substance has the status of deep illegality you can get away with punting shit gear, it’s called a captive market.

Back in the bad old days, when weed was Class ‘B’, everyone left dealing to the wide-boys and did their best not to get nicked by the rozzers. When weed was reclassified to Class ‘C’, everyone left dealing to the wide-boys and did their best not to get nicked by the rozzers. If weed is reclassified to Class ‘B’ again, I simply don’t know what we’ll do.

maninalift said,

The focus on psychosis seems to me odd. It is a relatively rare effect and as the discussions have shown rather hard to determine a causal link. What is more interesting, to me at least, is to understand the necessary impact of cannabis on the brain. You can tell someone who smokes heavily straight away, particularly teenagers, because of the slow speech and unnaturally lowered tone of their voice. My experience is that there is an impact on mental agility that lasts for at least two weeks. Is there a permanent effect after heavy use for a period of years? As a teenager ones brain is still undergoing a process of gross development, perhaps ridding it’s self of unnecessary connections, one might expect that this would be a dangerous time to be tampering with brain chemistry. But this is pure supposition.

I could hardly believe my ears when the BBC Radio reporter pointed out that people who are predisposed towards schizophrenia may also be more likely to use cannabis. How rarely do we hear such subtleties? I danced round kitchen and nearly spilled my wine.

superburger said,

but does the total dose of THC taken change when you smoke filthy lumps of soap-bar resin, or prime hydroponic skunk? has the mean intake of thc changed recently, or are people smoking fewer, stronger joints.

the fine/prison for possesion is the same whether you have are caught with skunk, or resin, so if you’re going to deal, you may as well deal the good stuff as the risk/reward ration is more favourable.

AnotherBee said,

You can have fun with the same sort of question by analysing the US suicide statistics. Over half US suicides make use of a firearm, therefore (if correlation equals causality) it can be argued that ownership of a firearm is a leading indicator of suicidal tendency.

“The ABSOLUTE risk is pretty low – out of 1000 people who use cannabis 4 more will develop psychosis than if those 1000 did not use cannabis.”

Is this claim actually justified by the study? I know they actually do a calculation along those lines but it seems a bit dodgy to me.

In particular, the assumption here is that 1% of people will get psychotic symptoms, but that is incorrect as 1% is the rate of schizophrenia, not psychotic symptoms which is up to 10%, or psychotic illness which is 2%(ish).

Secondly, the 1.4 odds ratio is calculated from psychotic symptoms, not schizophrenia, which less studies had data for, and where the odd ratio was 2.

Grathuln said,

As most will know the Government has commissioned the Advisory Council on the Misuse of Drugs (ACMD) to make further recommendations on the reclassification of cannabis. The Government has also put out a “public consultation” questionnaire, which can be downloaded from here: www.drugs.gov.uk/publication-search/drug-strategy/drug-strategy-consultation.pdf
The consultation ends 19th October, so get your comments in by then.

I have put in a freedom of information request to the ACMD for copies of the studies they will be reviewing; i.e. any new evidence they have on cannabis since the last study in 2005 recommended the classification remained as C. Others, probably more qualified than me, may like to obtain copies to review the quality of the science involved too.

Interestingly the Lancet study says: “In the Swedish
conscripts study, reverse causation was limited further
by analysis restricted to patients admitted for
schizophrenia at least 5 years after conscription; this
analysis produced similar results to the main analysis.”

richard: the dunedin one was a birth cohort study. they’re pretty amazing projects, you grab everyone who was born in a given place in a given time frame and follow them up at regular intervals, for all kinds of things, not just cannabis and psychosis. so they didn’t xplicitly search for a control group after finding some stoners, they were looking at a whole representative slice of population.

SleepyHead said,

The focus on psychosis in the popular press is a throwback to the hysteria-driven prohibitionist movement in the ‘States – Reefer Madness and films of its ilk portrayed yer average stoner in a manner not too dissimilar from that pilloried by Hunter Thompson: “You will not be able to see his eyes because of tea shades, but his knuckles will be white from inner tension and his pants will be crusted with semen from constantly jacking off when he can’t find a rape victim.”

Now where’s that porn site I was looking for?

Ambrielle said,

I’m in a birth cohort study. Going on *ahem* 31 years now. It’s fascinating stuff. I’ve undergone all sorts of tests, including whole body bone densitometry, dietary log books, glucose intolerance tests, and stress response and dexterity. I’ve also managed to get hold of all the published papers. I would expect this one to continue for at least another 30 years, since the original premise was to study heart disease risk factors. Who knows, the next adjunct study could well be a drug usage one. In which, of course, I will be in the control group ;).

gantlord said,

Read this week’s Economist and you’ll see that cannabis use has been steadily falling over recent years. Is it because of reclassification? Or has hash gone out of fashion? For the record, a schoolfriend was a heavy user of hash from 15 to 18 and experienced 2 psychotic/schizophrenic episodes while high. He stopped taking and no symptoms ever recurred. I’m inclined to believe that hash can trigger/cause/worsen schizophrenia if you take it enough and are prone. I think a lot of people here have a blindspot on this issue, and aren’t thinking objectively.

Gimpy said,

37. gantlord. you should avoid using anecdote as proof that beliefs are correct. However nobody disputes the credible evidence linking schizophrenia and cannabis in some individuals although the how and why has many unanswered questions. I have noticed that many cannabis users like to think it does them no harm which is how they justify its use so I think your right about the blindspot. Nevertheless many legal drugs are just as, if not more, harmful so the harm factor in itself is not grounds for illegality.

icarusfall said,

I had a friend who developed hypermania, and who smoked a hell of a lot of week at university. It’s completely anecdotal, though, and it’s a self-selecting sample (I had lots of other stoner friends as well, who all seem fine).

I don’t think anyone’s disputing that use is correlated with higher incidence of psychosis. It’s just not as dramatic an effect than the newspaper’s are implying. You’ve got to retain a sense of proportion about these things.

Gimpy said,

40. goodpoint icarusfall. Of course the ‘stoner blind-spot’ could be addressed by proper education and balanced debate and maybe even regulation. One of the things which seems to have been forgotten in all the schizophrenia debate is that cannabis is usually consumed in conjunction with tobacco. Separating consumption of the two could have a far greater beneficial effect on the health of the nation than scaremongering schizophrenia concerns. IMHO.

had to turn the forum off for a bit, it’s so server intensive – and i have a seriously massive server now – there was just no way it would work, it’ll be back on in a mo, sorry.

i dont think you get 200,000 random internet punters all looking at your swear words in the same hour very often. this will not happen again. think of it as proof that you are cool for being early adopters.

Dr Aust said,

Sigh. Another of those issues where whatever the scientific / medical truth, it will always be submerged by a ton of scare stories and political posturing.

As rs was sort of alluding to above, it seems to me that without detailed and accurate information on HOW MUCH cannabis the people in the studies get through the whole discussion is largely meaningless…

Also add the difficulty of controlling for “self-medication”, the problems of ruling out the (quite common) psychotic episodes that aren’t ultimately diagnosed as schizophrenia, and the “population factors” (dope smoking highest among adolescent males, earliest pyschotic episodes typical in the early 20s in men even if not smokers, etc etc)

…and you have a right mess.

I’m all for advising teenagers not to fry their brains daily with 2 hrs relentless toking on the hydroponic skunk, but the moral panic about the stuff is all too “reefer madness” for words.

My favourite isolated cannabis factoid is that the British Indian Civil Service compiled a seven volume, 3,281 page report on “Indian hemp” as far back as 1894.

In respect to the alleged mental effects of the drugs, the Commission have come to the conclusion that the moderate use of hemp drugs produces no injurious effects on the mind. It may indeed be accepted that in the case of specially marked neurotic diathesis, even the moderate use may produce mental injury. For the slightest mental stimulation or excitement may have that effect in such cases. But putting aside these quite exceptional cases, the moderate use of these drugs produces no mental injury. It is otherwise with the excessive use. Excessive use indicates and intensifies mental instability (1:264).”

Any comments from the pyschiatrists hereabouts as to whether we have notably changed this view (scientifically) in the last 113 years?

bludwulf said,

Hello
well it seems Dr Aust beat me to it , but I wanted to comment on the use of cannabis to self medicate for stress/anxiety problems.
I must say from personal experience that ranks right up there with “I just like the way it makes me feel” as reasons I’ve heard for smoking a lil grass.

Long time reader first time poster
I love your work Ben,
Keep it up!

Dr Aust said,

Obviously a lot of the terminology is different. The counter-point is that they were probably the last (only?) survey of this ever done by an official without it being a football of some perceived moral agenda or agendas.

It would be hard to argue with the view that, since Anslinger’s and others’ drug crusade in the States in the 20s and 30s, it has been pretty much impossible to have any sort of reasoned public or political discussion of “drugs of abuse” in general, and cannabis in particular.

So partly the question to those with professional experience in the “substances” field was (rephrased slightly): if we replace the outdated terminology of the 1894 Indian Civil service report with modern versions of the same concepts, how well do we feel the conclusions stand up?

I have had a go below, though as I am not a medical doctor, let alone a psychiatrist, the terminology is likely wrong. Corrections welcomed.

“In respect to the alleged mental effects of the drugs, the Commission have come to the conclusion that the moderate use of hemp drugs [i]does not cause or exacerbate mental difficulties[/i]. It may indeed be accepted that in the case of people with [i]more severe problems within the neurotic disorder spectrum[/i], even moderate [i]regular[/i] [cannabis] use [i]has the potential to cause exacerbation, [since] regular use of any mind-altering substance (e.g. alcohol) has the potential to exacerbate underlying problems in such relatively vulnerable individuals[/i]. But putting aside these quite exceptional cases, the moderate use of these drugs does not [i]damage mental health[/i]. Excessive use [is different] since [i]this is more likely to show co-morbidity with mental disorders.”[/i]

richard_p_auckland said,

I’d think it might be hard to control for other drugs, given that people would possibly be willing to be upfront about a widely accepted drug like cannabis, but not about a demonised (in NZ) substance like methamphetamine.

Though I seem to remember from Stats 101 that there’s a way around this – something about asking the subject to tick a box if they’ve either committed arson or they have a living grandmother. Since the latter can be predicted, you can extract the figure for the former.

atavachron said,

I had a number of ‘turns’ while smoking skunk – waking up in a state of abject undirected terror and dread. Then I stopped for a bit and it didn’t happen again – for a while. I think my own ‘psychosis’ had much more to do with my life being in the shitter than with how much THC I had in my blood stream. All anecdotal, carry on….

hass said,

man after reading all this it really made me light anthor joint..haha…i dont see why people shuldnt smoke cannabis i think the law shuld let it free and repect the one who smokes and the one who dont. thats my oppinion as a matter a fact am stonded right now thats why i find it real funny hahaha..and heres a shout for all those who smokes budzz “GOD BLESS YOU”